Tonight, we turn to a famous actor now making news. Fighting to hold onto his most cherished role ever. Far from any movie screen. This is a much different kind of role. The one as father. Jason patric is fighting for shared custody of his biological son named gus. But gus's mother says that was never part of the plan. And tonight, right here on "20/20," for the first time, jason patric sitting down with us, after learning california lawmakers have dealt him a serum use setback. &#9834; a cherished home video of a father and a son. But this famous hollywood actor is now starring in a real-lime d drama of his own. In a real life drama of his own. You may know him from his other roles, the ones on screen. Date he came from a famous family. Jackie gleason was his grandfather. But away from the glare, jason patric was working on something else, trying to have a baby with a woman he'd been dating on and off for years, danielle schreiber. I've been in a lot of relationships. I was always worried about having a child. I was with someone, someone that I trusted and I loved. And so I said, "well, we can try this route." But please, promise me that the ups and downs that come with it, you're not going to become the crazy person in that situation. I don't want it to affect who we are as a couple. Reporter: So, you were a little concerned that if we introduced a child into this -- the process more. Reporter: Danielle still wanted a baby and jason agreed to help her keep trying. She finally got pregnant through invitro. But danielle says there was never any agreement that he'd play the role of father. Did you say to her at the time, look, I'll help you with this, but I don't want to be the dad. No. Reporter: Would you had agreed to it had you known that danielle didn't want you a father? Never. The word parent means to beget, nourish or raise a child. That was my intense. Reporter: And the baby is born. The baby's born. Reporter: A little boy named gus. I can't describe it. Anyone who has had a child -- i was afraid to touch him because he was so small and everything. Reporter: That new baby seemed to draw the couple closer again. In jason's picture, they look like a family. You were at the house every day? Absolutely, yeah. I played music, I scratched his back, I helped him articulate his fingers. Speak shakespeare into his ear, because I think he should know that one day. Reporter: He showed us the home videos, their time spent together. But that would soon end and quickly. It was father's day, which happens to be your birthday. She gave me a big cake with gus on it. Reporter: That was the last time you were all together? Yes. Reporter: Because he says, soon after that birthday, after years of ups and downs they break up, but continue parenting together, it changed everything. One week later, I get an e-mail, speak to my attorney. I mean, I could have been knocked off my chair. It just blew me away. Reporter: You're convinced she was punishing without question. Reporter: Danielle was argue he was not a dad to her on sand the court ruled in her favor. My lawyer said, they're saying you're not the father. I said, what? You're not the father. Reporter: Their contention is because you went to that clinic, the law says -- the law says. Reporter: You're a sperm donor. Right. Reporter: If you are not married and go to a clinic, under the law, you're considered only a sperm donor, with no rights as a father. Two most important words in a child, two most important visions, ideas are momma and dada. It means safety, support. Food, love. Reporter: Danielle has also said that jason never intended to be a father. In fact, she says he insisted that it all be kept a secret. In the statement, she writes, i allowed jason to have contact with my son while we were dating and even then, he insisted i keep his donation a secret and uphold our original agreement. Show me the document that you have or e-mail where I say the world secret. Reporter: In fact, he points to this document instead, where at that invitro clinic, he signed his name next to the klein "intended parent." But has now learned that doesn't hold up in court. And apparently, neither do these images. Those home videos. Listen to the questions danielle asks her little boy. Which one is dada, right there? Uh-huh. Where's dada? That's right. Reporter: And jason patric points to a christmas card write by danielle. In her handwriting, jason, i love you more than anything, what else can I say. Then, for gus, dada, thank you for teaching me to pee in the toilet, watch airplanes, learn beatles songs. I love you dada, gus. Reporter: You point right to that card and say, that card proves that even danielle looked at you as the father. The whole thing is so absurd, david. Reporter: But we asked jason patric about one more thing. His name not on the birth certificate. So many people have pointed out the birth certificate, why your name wasn't put on there. If I can protect him from the bad aspects of my name that i can, I certainly will. This was my idea of trying, and it worked, for two years, there were no pictures of gus or me. None. Reporter: You thought by keeping your name off of the birth certificate that you were protecting gus from the media glare? Absolutely. Reporter: But tonight, that glare is on jason himself. Fighting for shared custody of that little boy. Just this week, testifying before california lawmakers, hoping they would change the law, that loophole, he says, that labels him simply a sperm donor and not a dad to gus. Sperm donors, a word that was never, ever mentioned in my house. A word I can guarantee you gus doesn't even though how to pronounce. Reporter: But law will makers put the debate on hold indefinitely. As we sit here, you know the number of weeks it's been. 26 on sunday. Reporter:26 weeks without seeing gus. No rights to visitation. Reporter: W what was the last thing you said to him? The last thing I said to him was, I picked him up, I put him in the car, we drove back, i drove himdanielles, I said, "i'm missing you, dada." I said, "that's all right. I'm going to see you tomorrow, little man." And I've never seen him again. Reporter: Can you hear his voice? No. Not anymore. And it breaks my heart. No. And he has a room in my house and I don't open the door. But he doesn't know me. You know, his little brow, wondering where his dad is, he doesn't know that. And he's not a possession. Reporter: And do you let yourself go there? I mean, do you ever wonder if you'll see him again? I'm going to fight until I'm dead to see him. And I'm doing things like this so he knows I've done everything in my power to make sure he has a dada. Reporter: What would you say to gus? I love you. This is not your fault. I promise you I'm going to do everything I can to see you. Reporter: And to danielle, if she's watching? Why? Why?

This transcript has been automatically generated and may not be 100% accurate.

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